One more minor simplification: W->region_showing may be only nil or t,
so it may be reduced to a bitfield.

Hard case:

   if (((!NILP (Vtransient_mark_mode)
	&& !NILP (BVAR (XBUFFER (w->buffer), mark_active)))
        != !NILP (w->region_showing))
       || (!NILP (w->region_showing)
	  && !EQ (w->region_showing,
		  Fmarker_position (BVAR (XBUFFER (w->buffer), mark)))))

There are two possible cases:

   - both w->region_showing and Fmarker_position (BVAR (XBUFFER (w->buffer), mark))
     are nil;
   - w->region_showing is t and Fmarker_position (BVAR (XBUFFER (w->buffer), mark))
     is a number, so !EQ (...) is always non-zero.

So if-statement may be simplified to:

   if (((!NILP (Vtransient_mark_mode)
	&& !NILP (BVAR (XBUFFER (w->buffer), mark_active)))
        != w->region_showing) || w->region_showing)

Dmitry